Using “at” in Mando is a little different

The word "zài " contains the entire meaning of "to be at/in".

What I am trying to impress on you here is we do not need to say,"wǒ shì zài xuéxiào" when we want to say, "I am at the school."

zài already contains all of the context of "to be at/in", it literally means all of that.

subject + zài+location + action	wǒ zài xuéxiào. → I (am) at school. 我在学校。
	Tā de bàba zài yīyuàn gōngzuò. → His dad at hospital works (”his dad works at the hospital”) 他的爸爸在医院工作。